Nanoparticulate cerium oxide (nanoceria) has many current applications as well as potential future applications. It is well known as an important component in solid oxide fuel cells, three-way automotive exhaust catalysts and automotive fuel borne catalysts. Its utility is often attributed to its redox chemistry, resulting from the facile Ce3+/Ce4+ electrochemical conversion. This allows nanoceria to store oxygen under oxidizing conditions (forming Ce4+) and to release oxygen under reducing conditions (forming Ce3+ and oxygen vacancies), a property commonly referred to as its oxygen storage capacity (OSC).
An end use application to which this invention particularly relates is the automotive fuel borne catalyst, a technology offering the potential, in the case of diesel engines, of fuel efficiency gains of about 34%. It is well recognized that a faster burn of fuel within the diesel combustion chamber will produce a higher pressure resulting in more energy capture as mechanical work and less energy waste as heat, thereby providing improved fuel economy. Moreover, reductions in harmful emission gases (e.g. NOx, CO, CO2, and soot) have also been observed when nanoceria is included as an additive in diesel fuel. These benefits are believed to result from the ability of nanoceria to store and release oxygen (OSC) in a diesel engine combustion chamber, thereby reducing local inhomogeneities in the fuel/oxygen mixture, enabling a faster and more complete burn.
Although substantially pure cerium oxide nanoparticles are of some benefit when included in applications such as fuel additives, it may be even more beneficial to use cerium oxide doped with components that may result, in part, in the formation of additional oxygen vacancies. Herein, the term “doped particle” refers to a particle containing one or more foreign or dopant ions. Doping of cerium oxide with metal ions to improve ionic transport, reaction efficiency and other properties is disclosed in, for example, U.S. Pat. Nos. 6,752,979; 6,413,489; 6,869,584; 7,169,196 B2; 7,384,888B2; and U.S. Patent Appl. Publ. No. 2005/0152832.
The homogeneous dispersal (doping) of two or more atomic species (metals) in ceria is disclosed by Talbot et al. in U.S. Pat. No. 6,752,979 using a surfactant micelle technique. Other literature includes, for example, Harrison et al. Chem. Mater. 2002, 14, 499-507 who describe Cu and Cr doping of ceria by a variety of methods followed by calcining; Liu in Chinese Journal of Chemical Physics 20, 6 (2007) who examine NiO and Bi2O3 doped CeO2; US 20060120936A1 claims a three component system with Ce as the first component, Cu, Co, or Mn as the second component, and Sr as the third component. A process for predominately surface doping of cerium oxide nanoparticles is described by Wakefield in U.S. Pat. No. 7,169,196; wherein adsorption of dopant ions onto the surface of the nanoparticles is followed by firing (i.e. conventional high temperature ceramic processing), which will result in an uneven dopant distribution. Use of doped 10-20 nm diameter cerium oxide nanoparticles as a fuel additive is described, for which copper doping is particularly preferred.
Scientific focus around CeO2 core and shell structures is brought to bear by Omata, et al. in the Journal of the Electrochemical Society 2006, 153(12) A2269-A2273, wherein CeO2/ZrO2 core-shell nanocrystals were synthesized by the addition of undoped CeO2 nanocrystals as seed crystals in the ZrO2 source solution, followed by reaction at 300° C. A core-shell nanostructure is proposed, consisting of an undoped 2.4 nanometer (nm) diameter CeO2 core with a 1.2 nm thick ZrO2 shell, and is supported by XRD, high-resolution TEM and XPS results. This proposed core constitutes about 12.5% of the nanoparticle by volume, while the shell constitutes about 87.5% of the nanoparticle by volume. In other work, Singh, P. and Hegde, M. S. employ a hydrothermal method using diethylenetriamine and melamine as complexing agents, describing that the cubic fluorite lattice of ceria is still evident by EXD at up to 50% substitution of Zr for Ce, in Journal of Solid State Chemistry 181 (2008) 3248-3256. Lambrou, P and Efstatiou, A. in Journal of Catalysis, 240 (2006) 182-193 report an increase in OSC for doped ceria in going from 0.1 to 0.3% iron content, but then a loss in OSC at 0.4% relative to the lower iron levels.
Zirconium doping has received much attention due to its ability to inhibit the sintering of CeO2 at high temperature, as disclosed, for example, in U.S. Pat. Nos. 6,051,529 and 6,255,242 B1 Umemoto et al. (2001); U.S. Pat. No. 6,387,338 B1 Anatoly, et al. (2002); U.S. Pat. No. 6,585,944 B1 Nunan et al. (2003); and US 20070197373 Miura, M. et al. (2005). Kuno, O. in U.S. Pat. No. 7,384,888B2 discloses an undoped CeO2 core surrounded by a ZrO2 shell made by addition of Zr to a preformed ceria sol, followed by calcination at 700° C. Additional structured core and shell art is provided by WO 2004/052998A1, US 2006/0138087, DE 2001-101311173A3, DE 2001-10164768, U.S. Pat. Nos. 6,136,048A1, 5,500,198 and WO0200812A2. However, none of these patents or patent applications teach how to obtain particle sizes of less than about 5 nm, or less than 3 nm for the doped particles.
Several workers have suggested that a homogeneous dopant spatial distribution is preferred. Mamontov et al. J Phys. Chem. B 2003, 107 13007-13-14 concluded on the basis of pulsed neutron diffraction studies that in Ce0.5Zr0.5O2 a more homogeneous distribution of Zr was responsible for the enhanced OSC and that OSC did not correlate with particle or crystallite size. Nagai et al. in Catalysis Today, 74, (2002) 225-234 arrived at the same conclusion as Mamontov using EXAFS techniques. On the topic of particle size distribution, Rohart, E. et al. in Topics in Catalysis Vols. 30/31, 417-423 (2004) conclude that a fractal (heterogeneous size distribution) texture is preferred for thermal stability after examining a range of Zr and Ce compositions.
Commonly assigned PCT/US07/077,545, METHOD OF PREPARING CERIUM DIOXIDE NANOPARTICLES, filed Sep. 4, 2007, describes stabilized cerium oxide nanoparticles comprising a core and a shell, wherein the shell comprises a material selected from the group consisting of a transition metal, a lanthanide, a sulfur-containing compound that may include a mercaptide group, and combinations thereof. Preferably, the core comprises about 90% or less of the nanoparticle by volume, and the shell comprises about 5% or more of the nanoparticle by volume. The disclosure states that the core of the particle preferably includes at least about 75%, more preferably, about 95% or greater of the bulk particle, and may be optionally doped with a metal. The shell, including the outer portion and surface of the particle, preferably comprises about 25% or less, more preferably about 10% or less, most preferably about 5% or less, of the particle, and includes a transition or lanthanide metal.
In summary, it is clear that significant opportunities for improvement of nanoceria based fuel additives remain. To date, relative to the theoretical potential gain of about 34%, only modest diesel fuel efficiency gains of 5-10% have been reported in laboratory tests of nanoceria additives, while commercial on-road diesel bus fuel efficiency gains of only about 4-5% have been realized. In addition, lengthy diesel engine conditioning periods on the order of 8 weeks have been required before the fuel efficiency benefits of nanoceria fuel additives have been observed. Lastly, nanoceria fuel additives have failed to show benefits in gasoline engines, wherein it is believed that oxygen is not released quickly enough from the nanoceria particles to be effective at the higher rpm's (shorter combustion times) at which these engines typically operate. Thus there remains a need to further increase the amount of oxygen stored/released, as well as the rate at which oxygen is stored/released by nanoceria particles when used as a fuel borne catalyst. Means to simultaneously and independently control of both the thermodynamic (OSC) and kinetic (Rate Constant) properties of nanoparticle combustion catalysts, not heretofore achieved, would be greatly beneficial.